1. Field of the Invention
The present invention relates to a transfer fixing member, a transfer fixing device and an image forming apparatus using the transfer fixing device in a transfer fixing system, typified by copiers, printers, facsimile or complex machines thereof.
2. Description of the Related Art
Image forming apparatuses, in each of which an image is formed on an image bearing member by means of a developing unit, the image on the image bearing member is primarily transferred onto an intermediate transfer medium by means of a primary transfer unit, the image on the intermediate transfer medium is secondarily transferred onto a recording medium by means of a secondary transfer unit and the image on the recording medium is then fixed by means of a fixing unit, have been widely known. As the fixing unit, a pressurizing member is contacted with a fixing member containing a heat source inside, and the contact position is formed as a fixing nip where the recording medium is held and conveyed as well as heated. As the fixing members, rollers and belts are used, and as the pressurizing members, rollers, belts and fixed pads are used.
Images to be transferred include not only images of single color, but also of images of multi-color, such as full-color. Generally, a powder mainly consisting of a resin and having an electrostatic property, a so-called toner is used for forming images. With regard to fixing of these images, fixing property, particularly, temperature property in accordance with modes of images to be transferred is important. The temperature property affects heat transfer between a toner and a recording medium. The heat transfer direction changes depending on the temperature of a toner surface which is in contact with a fixing member and the temperature of a surface of a recording medium which is in contact with the toner (interface temperature). Of the temperature properties, the temperature of the toner surface affects glossiness required for a full-color image and the like. The temperatures of the toner and the surface of the recording medium in contact with the toner affect penetrance (adhesiveness) of the toner relative to the recording medium.
A fixing device of a full-color image forming apparatus is shown in FIG. 1. In the fixing device, image bearing members A to D capable of forming images of respective colors are arranged in parallel, and an intermediate transfer medium E, which corresponds to a primary transfer member and has an extended surface along the direction of the parallel arrangement, is disposed, and the images of respective colors are transferred sequentially to the intermediate transfer medium E. A transfer device F is disposed as a secondary transfer member for simultaneously transferring superimposedly transferred images to a recording medium so as to face and contact with the intermediate transfer medium E. And then, a recording paper on which images are simultaneously transferred is conveyed toward a fixing device G.
The fixing device G as shown in FIG. 1 has a configuration which employs a heat roller fixing system in which a fixing roller G1 and a pressurizing roller G2 face and contact with each other to form a fixing nip having a distance L, and an unfixed image on the recording paper is fixed by heat generated from the fixing roller G1. The heat roller fixing system is frequently used recently, as it is advantageous for achieving higher speed because of its high heat efficiency, for obtaining stable fixing efficiency because of its high heat transfer efficiency and for its simple structure because it is usable as a conveying medium for recording media.
A warming-up operation is performed in the fixing device G until the fixing nip reaches a predetermined temperature. In the case of a full-color image, approximately 1.5 times of heat quantity is required because the thickness of a superimposed toner image is thicker than that of a single-colored image such as black and white image. Thus, the heat quantity applied to the recording medium tends to increase, compared to the case when a single-colored image is formed. As a result, the recording medium is likely to be excessively heated, and when many full-color images are fixed at high speed, there is a possibility that electrical power for heating may become insufficient to sustain operation of the image formation with power capacity of power sources for business use such as 100V and 15 A, etc.
Excessive heating causes the recording medium itself to be excessively heated. Such phenomenon does not conform to the users' intention when handling recording media, and when a toner is softened again by excessively heating, stacked recording media are attached to each other, namely, stack on one another, resulting in poor workability, for example, the recording media have to be peeled off when being taken out. As for the failures due to excessive heating, when a recording medium such as the one of which surface is coated specially for preventing blurring is used for forming an image by mistake, instead of a recording medium such as a regular paper on which a toner is transferred, a coating material is transferred to a fixing member by heat, that is, offset is likely to occur, and smear or winding of recording media tend to occur at the fixing member. Thus, normally unnecessary operation for an image forming apparatus, such as removing the winding recording media or cleaning the fixing members, may be needed and it is disadvantageous in terms of workability.
In such a conventional image forming apparatus, image quality tends to be decreased in a step of transferring an image to a recording medium. In an apparatus which uses an electrophotographic method for image forming, images are transferred to a recording medium electrostatically by applying electric bias from the backside of the recording medium. In this case, a paper is mainly used as a recording medium. There are various papers, such as papers having various thicknesses, for example, a regular paper, cardboard and the like, and papers having surface properties of high quality, rough and the like. Because electric properties of the recording medium tend to change depending on the conditions such as hygroscopic property, thickness, surface property (irregularities), and the like of the recording medium, it is difficult to maintain constant transfer property when images on the image bearing member are transferred to the recording medium directly or through an intermediate transfer medium, and abnormal images are easily formed. Specifically in the case of a paper having rough surface property, because an intermediate transfer medium cannot follow the surface property of the paper, microscopic gaps are formed, where abnormal discharge occurs, and then an image is not transferred normally and a rough image is easily formed. Thus, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2004-93864, a method of defining a surface roughness and elastic modulus of the outermost layer of an intermediate transfer medium so as to prevent from forming a microscopic gap is proposed. However, individual toner has a small particle diameter, compared to the irregularities of a surface of paper as a recording medium. Thus, toner scattering in intermediate transfer and image roughness may not be prevented depending on a surface roughness of a paper.
Next, images transferred to the recording medium are heated in a fixing device, and the temperatures differ in a toner layer thickness direction during fixing. In other words, in the case of the configuration as shown in FIG. 1, heating begins at a point where the images reach the fixing device G for the first time, and the temperature of the toner on the surface layer side opposite to the interface of the recording medium is considerably lower than that on the interface side of the recording medium in the toner thickness direction, thereby increasing temperature gradient in the toner thickness direction.
The fixing temperature may be increased in order to solve the above problem, however, when a heating temperature is increased, heating burden (increase in power consumption) increases, and the recording medium tends to be excessively heated as described above. Thus, problems caused by the excessively heated state of the recording medium or resoftening the toner are not solved.
Meanwhile, currently the image forming apparatuses in which all steps are performed stepwisely as mentioned above are the mainstream in the market. However, it is also known that an image forming apparatus which includes a so-called transfer fixing unit configured to perform a transfer step and fixing step simultaneously as disclosed in JP-B No. 3042414 and JP-A No. 2004-145260. JP-B No. 3042414 discloses an image forming apparatus in which an image is transfer-fixed from an intermediate transfer medium to a recording medium. JP-A No. 2004-145260 discloses an image forming apparatus in which an image is secondarily transferred and fixed from an intermediate transfer medium to a transfer fixing member, and then tertiarily transferred and fixed from the transfer fixing member to a recording medium. In these mentioned techniques, a powder mainly consisting of a resin and having an electrostatic property, a so-called toner is generally used for forming an image.
Because transferring and fixing are performed simultaneously in the image forming apparatus including the transfer fixing unit as described above, the degradation of image quality is least likely to occur even when a paper having rough surface property is used. This is because an image is simultaneously heated and transferred, and the toner is softened and melted by heat to become a viscoelastic block-shaped mass, making easy for the image to be transferred even in a microscopic gap of paper. Because of these advantages, the image forming apparatus having a transfer fixing unit is suitable for forming high quality images.
However, transfer fixing ratio is low and graininess is inappropriate for a highlight area in the transfer fixing step. In other words, it is known that a sufficient amount of toner is not easily transferred to a recording medium and images are not improved and sometimes may be poor, compared to normally-operated electrostatic-transfer system. Moreover, it has been found that when energy for transfer fixing is increased in order to improve the transfer fixing ratio and image quality of the highlight area. However, problems of nonuniformity in fixing and glossiness may occur in a high-density area containing a large amount of toner because the toner is excessively fixed, although the transfer fixing ratio is excellent even in the highlight area. Similar to the problem in transferring an image from the intermediate transfer medium to the paper, the problem occurs because the transfer fixing member does not sufficiently follow a paper surface, and a dot image at high-light do not contact with the paper, or contact therewith only in a small area.
Like the conventional image forming apparatus configured to perform a fixing step, in the image forming apparatus configured to perform a transfer fixing step, a photoconductor, intermediate transfer medium and transfer fixing member respectively need to have appropriate resistivities to perform electrostatic transfer. For example, when an insulator is used for the transfer fixing member, the transfer fixing member is charged, images are continuously transferred and appropriate electrostatic transfer cannot be performed.
According to JP-A No. 2004-93864, an appropriate range of a volume resistance of an intermediate transfer medium is 107 Ω·cm to 1015 Ω·cm. An average volume resistance “RV” between an electrode which applies a primary transfer bias and a surface of an intermediate transfer belt is 107 Ω·cm to 1013 Ω·cm, and an average resistance “RS” of the surface of the intermediate transfer belt is larger than RV. Thus, both high effective transfer and prevention of toner scattering in transferring can be improved. Moreover, both the high effective transfer and prevention of toner scattering in transferring can be improved on a further high level by making an average resistance “RS” of the back surface of the intermediate transfer belt higher than RV. A conductive material is dispersed to adjust the resistances.
However, when the conductive material is dispersed in the transfer fixing member so as to adjust its resistivity to an appropriate resistivity of the intermediate transfer medium, the conductive material dispersed therein causes offset in the transfer fixing member which is heated up to a fixing temperature of a toner in transfer fixing. Thus, it is difficult to adjust the resistance. It has been understood that an appropriate image cannot be obtained when images are continuously printed because the transfer fixing member is charged. Moreover, JP-B No. 3021352 and JP-A No. 2006-221066 are also related arts of the present invention.